Two different hydrogenase enzymes from sulphate-reducing bacteria are responsible for the bioreductive mechanism of platinum into nanoparticles

A mixed consortium of sulphate-reducing bacteria was used to investigate the enzymatic mechanism for the total bioreduction of platinum (IV) into platinum (0) nanoparticles. It was established that two different hydrogenase enzymes were involved. First the platinum (IV) was reduced to platinum (II) by a two-electron bioreduction using an oxygen-sensitive novel cytoplasmic hydrogenase. Second the platinum (II) ion was reduced to platinum (0) nanoparticle by another two-electron bioreduction involving an oxygen-tolerant/protected periplasmic hydrogenase. The enzyme was identified from its reaction with Cu(II), an active inhibitor of periplasmic hydrogenases. No exogenous electron donors were necessary as endogenous production of hydrogen/electrons, via the oxidation of metabolites, was generated in situ by the cytoplasmic hydrogenase. The hydrogen then dispersed through the cell to the periplasm where it became available for use by the periplasmic hydrogenase. The endogenous electrons were used, in the absence of sulphate, for the reduction of platinum (II) by the periplasmic hydrogenase. It was found that the Pt(IV) ion must be fully reduced before reduction of the Pt(II) ion would begin. Transmission electron microscopy and energy dispersive X-ray analysis confirmed the deposit of platinum particles into the periplasmic space.